1. Field of the Invention
The present invention relates to a polishing method as one semiconductor fabrication technique and an apparatus for performing the method and, more particularly, to a polishing method and a polishing apparatus for conductor films.
2. Description of the Related Art
Recently, many large-scale integrated circuits (LSIs) formed by integrating a large number of, e.g., transistors and resistors on a single chip are used in main parts of computers and communication systems. For this reason, the performance of an entire system of this type largely relies upon the performance of an individual LSI. As the packing density of an LSI has been increased, various micropatterning techniques have been developed. The minimum processing size of patterns has been decreased year by year and has already become the submicron order at present. A CMP (Chemical Mechanical Polishing) technique is one of the techniques developed to meet these strict requirements for miniaturization. This technique is essential in performing, e.g., planarization of insulating interlayers, formation of plugs, formation of buried metal interconnections, or isolation of buried elements, in semiconductor device fabrication processes.
FIGS. 1A, 1B, 1C, 1D, 1E, 1F and 1G are sectional views showing an example of formation of buried metal interconnections using a conventional polishing technique. In these drawings, only metal interconnections are illustrated, and all of other structures such as elements below an insulating film 2 are omitted. First, as shown in FIG. 1A, the insulating film 2 is formed on a semiconductor substrate 1 such as a silicon substrate, and the surface of the film 2 is planarized. Subsequently, as shown in FIG. 1B, trenches for interconnections or openings for connecting lines are formed in the insulating film 2 by using regular photolithography and etching processes. Thereafter, as shown in FIG. 1C, an interconnecting metal film 3 with a thickness larger than the depth of the trenches formed in the insulating film 2 is formed. In this case, to prevent an interdiffusion or a reaction between the insulating film 2 and the interconnecting metal film 3, a barrier metal film can also be formed between them.
Subsequently, to leave the interconnecting metal film 3 behind in only the trenches or the openings, polishing is performed for the film 3 by using alumina particles or the like as polishing particles. In this case, a film consisting of a substance having a high polishing velocity selectivity with respect to the interconnecting metal film 3 can be formed as a polishing-resistant film either above or below the film 3.
Subsequently, as disclosed in the methods invented by Wada et al. (Japanese Patent Application Nos. 4-065781, 4-212380, 4-269202, and 5-67410), annealing is performed continuously in a vacuum from the state shown in FIG. 1C by using an Al film as the interconnecting metal film, thereby forming a single crystal of Al in the trenches and at the same time selectively leaving the Al film behind on projecting portions (where no trench is formed) of the insulating film 2 as shown in FIG. 1D. Thereafter, as shown in FIG. 1E, the remaining Al film is removed by polishing.
As illustrated in FIG. 1E, an ideal shape after the polishing is that the interconnecting metal film 3 remains only in the trenches, metal interconnections (connecting lines) are formed with neither flaws nor impurities on the surface, the interconnecting metal film 3 does not exist on the projecting portions at all, and the surface of the insulating film 2 is at the same level as the surface of the interconnecting metal film 3.
In an actual polishing process, however, the surface to be polished of the interconnecting metal film 3 is damaged and roughened by a mechanical action between the surface of the metal film and polishing particles or between the surface and a pad for holding a polishing agent, or polishing particles are buried in or left behind on the interconnecting metal film 3. In addition, as shown in FIG. 1F, a phenomenon termed dishing takes place in which the thickness of a central portion decreases in particularly a wide region of the interconnecting metal film 3 buried in the trenches or openings. This tendency appears conspicuously when a metal having a low hardness and a high ductility, such as Al or Cu, is used as the material of the interconnecting metal film 3. The occurrence of flaws or dishing, or the residue of polishing particles on the surface of an interconnecting metal film increases the resistance of resulting interconnections or causes disconnections, leading to a decrease in reliability or in product yield.
An example of polishing that can prevent the occurrence of flaws or dishing on the surface of an interconnecting metal film is, if a film to be polished is an Al film, a treatment using a slurry prepared by dispersing alumina particles in an aqueous acidic solution with a pH of 3 or less (Beyer et al., U.S. Pat. No. No. 4,944,836). In this polishing, however, the polishing rate ratio (SiO.sub.2 /Al) of SiO.sub.2 to Al cannot be satisfactorily decreased. Therefore, if this polishing is applied to the formation of buried interconnections, the SiO.sub.2 film (insulating film 2) is also polished at the same time as the Al film (interconnecting metal film 3) is polished. This decreases the depth of trenches formed in the SiO.sub.2 film, resulting in decreased dimensions of interconnections. In addition, this polishing has another problem of corrosion of a polishing apparatus resulting from the use of the aqueous acidic solution.
On the other hand, a method using no polishing particles is reported (International Electron Devices Meeting Technical Digest 1992, p. 976, Y. Hayashi et al.), in which aluminum plugs are formed by polishing using a solution mixture of an amine and hydrogen peroxide water. In this method, since no polishing particles are used, the polishing proceeds nearly chemically, and neither flaws nor roughness rarely takes place in aluminum. However, dishing occurs significantly because the ratio (polishing velocity/dissolution velocity) of the polishing velocity of aluminum to the dissolution velocity of aluminum with respect to the solution mixture is low. For example, if this polishing is performed for a sample in the state shown in FIG. 1D, the Al film (interconnecting metal film 3) formed in the trenches to serve as interconnections vanishes before the Al film remaining on the projecting portions are removed, as shown in FIG. 1G.